Abstract A first order quasi-chemical treatment of binary interstitial solid solutions has been given. The enumeration of the nearest neighbor pairs is simplified by only considering those pairs whose number depends on the atomic configuration of the solution. Simple expressions have been derived for the partial Gibbs free energy and thermodynamic activity of an interstitial solute atom in terms of the pairwise interaction energy between two nearest neighbor solute atoms. It is shown that the equations yield the appropriate results in the limits when the binding energy between solute pairs becomes zero or infinite. Finally the equations are shown to be compatible with the known activity of carbon in austenite and a computer fitting technique has been used to derive the value of the solute-solute binding energy concomitant with the closest fit of the experimental data fo the theoretical activity equation.
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